The invention relates to corrosion resistant rolling element bearings and a process for producing races for such bearings.
Some bearing applications require bearings which are capable of both enduring high loads and surviving in very corrosive environments. For example, so-called airframe bearings, such as, the bearings on which the control surfaces and flaps of aircraft oscillate, must survive exposure to moisture and deicing fluids, not to speak of salt spray on occasions. Moreover, these bearings experience wide variations in pressure which cause them to ingest fluids, bringing those fluids into contact with the raceways which deteriorates the raceways. Bearings for machinery used in the food processing industry likewise operate in hostile environments characterized by aqueous corrosion. The high strength material from which high load bearings are typically made (e.g., 52100 bearing steel) does not provide the required level of corrosion resistance for such environments.
In an effort to improve the corrosion resistance of such bearings, other base materials, such as 316 stainless steel, have been utilized. A problem with many such alternative base materials, however, is that they are not hardenable and thus are not capable of providing the required load handling capabilities of the high strength steels. Other stainless steels, such as 440C stainless, are hardenable, but do not have sufficient resistance to corrosion. Thus, insofar as the stainless steels are concerned, they are either corrosion resistant and incapable of acquiring suitable hardness or else capable of being hardened and incapable of resisting corrosion.
Another approach has been to deposit Thin Dense Chrome (TDC), that is, a very hard plating of chromium onto, the exposed areas including the wear or functional surfaces, such as the raceways along which the rolling elements roll. With TDC, however, it is very difficult to obtain sufficiently thick layers while still achieving the required level of consistency, that is, an absence of holes and surface flaws which provide focal points at which corrosive activity tends to occur. In this regard, chromium is noble to steel in most corrosive environments, and thus any break in the chromium coating will cause the steel to corrode at that break. Hence, the chromium must form a perfect physical barrier.
Yet another approach has been to deposit cadmium protective layers, which are soft in comparison to the hardened, high strength steel from which the bearing is constructed. Due to its softness and other characteristics, cadmium is not well suited for use on the functional surfaces. Under load conditions, the cadmium may separate from the steel base material and interfere with the operation of the bearing or otherwise is quickly worn off, thereby eliminating the physical and galvanic protection which it originally provided. As a consequence, in cadmium plated bearings, the cadmium does not exist along the functional surfaces, but instead the steel is exposed at these surfaces. Apart from that, the plating solution from which cadmium is derived also contains cyanide which is extremely toxic. Environmental regulations do not favor cadmium plating by reason of the toxicity of the plating solution.
Most processes for plating steel rely on electrochemical reactions within plating solutions that contain and indeed often liberate hydrogen. During the process the steel absorbs hydrogen, and the hydrogen embrittles the steel. But a measure of ductility, not brittleness, is desired in bearing races and the rolling elements which move along them. Cadmium deposits on steel in a somewhat porous condition, and one can relieve hydrogen embrittlement simply by baking the steel part after it is plated. During baking the hydrogen escapes through the pores in the coating. Some other metals deposit on steel in a generally impervious condition and in effect trap hydrogen in the steel so that it cannot be easily driven off by baking. Zinc and traditional zinc alloys have exhibited this characteristic when deposited by conventional electro-plating processes.